Image sensor chips, which include front side image sensor chips and Backside Illumination (BSI) image sensor chips, are widely used in applications such as cameras. In the formation of image sensor chips, image sensors (such as photo diodes) and logic circuits are formed on a silicon substrate of a wafer, followed by the formation of an interconnect structure on a front side of the wafer. In the front side image sensor chips, color filters and micro-lenses are formed over the interconnector structure. In the formation of the BSI image sensor chips, after the formation of the interconnect structure, the wafer is thinned, and backside structures such as color filters and micro-lenses are formed on the backside of the silicon substrate. When the image sensor chips are used, light is projected on the image sensors, in which the light is converted into electrical signals.
The image sensors in the image sensor chips generate electrical signals in response to the stimulation of photons. The full well capacity and the signal-to-noise ratio of the image sensors are preferably high. To increase the full well capacity and the signal-to-noise ratio of the image sensors such as photo diodes, the impurity concentrations of the p-type and/or n-type regions of the photo diodes may be increased. Such increase, however, results in the degradation of some other performance of the image sensors. For example, the dark current and white pixel performance may be adversely increased due to the increase in the impurity concentrations. Furthermore, the leakage currents of the photo diodes may be increased due to higher impurity concentrations, wherein the leakage currents occur between the photo diodes and the adjacent isolation regions.